Migrate chat to AI SDK v5

[iKoles-dev]

/claim #88

Summary

• upgrade AI SDK core, React, provider, OpenRouter, and compatible Zod dependencies
• migrate chat transport and API streaming from v4 data streams to v5 UI message streams
• update UI message parts, tool rendering, attachments, persistence, sync, and generated image media types for v5 contracts

Validation

• COREPACK_ENABLE_STRICT=0 pnpm run format:check
• NEXTAUTH_URL=http://localhost:3000 AUTH_SECRET=development-secret DATABASE_URL=postgresql://user:pass@localhost:5432/toolkit COREPACK_ENABLE_STRICT=0 pnpm run check
• NEXTAUTH_URL=http://localhost:3000 AUTH_SECRET=development-secret DATABASE_URL=postgresql://user:pass@localhost:5432 COREPACK_ENABLE_STRICT=0 pnpm run build-pure